This invention relates to the field of lasers and more specifically to the field of injection lasers.
Present silica-based optical fibers can be fabricated to have a loss in the 1.3-1.6 micron wavelength region which is an order of magnitude lower than the loss occurring at the 0.85 micron wavelength of present lightwave communications systems, e.g., 1/4 db/km versus 2-3 db/km. Furthermore, these fibers can be fabricated to have a transmission delay distortion in the 1.3-1.6 micron wavelength region which is two orders of magnitude lower than the transmission delay distortion at 0.85 microns, e.g., 1-2 ps/km nm versus 100+ ps/km nm. Thus, the dispersion-limited transmission distance for high bit rate lightwave communications systems can be maximized by using a single-frequency, i.e., single-longitudinal-mode, injection laser generating output at the 1.55 micron wavelength where the fibers have minimum loss. For these reasons, present efforts in the development of lightwave communications systems are aimed at the 1.3-1.6 micron wavelength region instead of at the wavelength region surrounding 0.85 microns.
InGaAsP injection lasers produce output in the desired 1.3-1.6 micron wavelength region. However, typical single-resonator InGaAsP injection lasers have a laser cavity length in the 250-300 micron range. This results in mode spacing between 6 and 9 Angstroms. Since the gain spectral width of InGaAsP injection lasers is approximately 250-300 Angstroms, there are more than 30 longitudinal modes under the gain spectral width of a 250 micron long laser. Thus, the gain difference between modes is small and mode discrimination between the main mode and side modes is poor in InGaAsP injection lasers.
The injection laser, like all other oscillators, is perturbed by internal random processes which cause its output to fluctuate. One example of this, known as mode-partition-noise, is the fluctuation at turn-on in the relative intensities of various laser modes while the total output power of the laser remains fixed. Mode-partition-noise is a consequence of random fluctuations in the photon densities of the various modes at the moment threshold is reached.
A great deal of time has been expended in an effort aimed at decreasing mode-partition-noise. These efforts generally involve the use of lasers with improved mode selectivity. However, even if the mode-partition-noise is substantially decreased, main mode power fluctuations in the laser output (even in a dc operated laser) impose an ultimate limit on the performance of any lightwave communications system. Experimental observations indicate that power dropouts frequently occur in the main spectral line of dc-biased injection lasers having nearly single-longitudinal-mode behavior. These dropouts can cause errors in a communications system when the ratio of the main mode to side mode power is less than 50:1.
Main mode power fluctuations generally result from at least two significantly different physical mechanisms: (1) as a direct result of main mode photons interacting with the carrier density; i.e., a given photon may stimulate another photon or be absorbed and (2) as a result of gain fluctuations driven by carrier-density fluctuations, the latter resulting from photon-electron interactions of all modes--i.e., the main mode and all side modes--interacting with the carrier density. The side mode photon density interacts with the main mode photon density only through the carrier density; however, because the variance of the side mode photon density is far larger than that of the main mode (a consequence of the very small number of photons in the side mode), the side mode influence on the carrier density can be significant even though the mean value of the side mode power is low. The second mechanism can bring about mode dropouts when the mean value of the side mode power is large enough so that its fluctuating value can be larger than the fluctuating value of the main mode power, with enough joint probability to be significant.